[PATCH v6 09/23] sched/core: Push current task from non preferred CPU

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Actively push out task running on a non-preferred CPU. Since the task is
running on the CPU, need to stop the cpu and push the task out.
However, if the task is pinned only to non-preferred CPUs, it will continue
running there. This will help in maintaining the userspace affinities
unlike CPU hotplug or isolated cpusets.

Though code is similar to  __balance_push_cpu_stop and quite close to
push_cpu_stop, it is being kept separate as it provides a cleaner
implementation with CONFIG_PREFERRED_CPU.

Add push_task_work_done flag to protect work buffer.
Works only with FAIR class.

For now, only current running task is pushed out. This keeps the code
simpler. In future optimization maybe done to move all the queued
task on the rq.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/core.c  | 87 ++++++++++++++++++++++++++++++++++++++++++++
 kernel/sched/sched.h |  8 ++++
 2 files changed, 95 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index aa4201bb8082..56905bac9525 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5797,6 +5797,9 @@ void sched_tick(void)
 	unsigned long hw_pressure;
 	u64 resched_latency;
 
+	if (!cpu_preferred(cpu))
+		sched_push_current_non_preferred_cpu(rq);
+
 	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
 		arch_scale_freq_tick();
 
@@ -11315,3 +11318,87 @@ void sched_change_end(struct sched_change_ctx *ctx)
 		p->sched_class->prio_changed(rq, p, ctx->prio);
 	}
 }
+
+#ifdef CONFIG_PREFERRED_CPU
+/* npc - non preferred CPU */
+static DEFINE_PER_CPU(struct cpu_stop_work, npc_push_task_work);
+
+static int sched_non_preferred_cpu_push_stop(void *arg)
+{
+	struct task_struct *p = arg;
+	struct rq *rq = this_rq();
+	struct rq_flags rf;
+	int cpu;
+
+	/* sanity check and clear */
+	if (cpu_preferred(rq->cpu)) {
+		scoped_guard (rq_lock, rq)
+			rq->push_task_work_done = 0;
+		put_task_struct(p);
+		return 0;
+	}
+
+	raw_spin_lock_irq(&p->pi_lock);
+
+	/* This could take rq lock. So call it before rq lock is taken */
+	cpu = select_fallback_rq(rq->cpu, p);
+	rq_lock(rq, &rf);
+	rq->push_task_work_done = 0;
+	update_rq_clock(rq);
+
+	context_unsafe_alias(rq);
+
+	if (task_rq(p) == rq && task_on_rq_queued(p))
+		rq = __migrate_task(rq, &rf, p, cpu);
+
+	rq_unlock(rq, &rf);
+	raw_spin_unlock_irq(&p->pi_lock);
+	put_task_struct(p);
+
+	return 0;
+}
+
+/*
+ * Push the current task running on non-preferred CPU.
+ * Using this non preferred CPU will lead to more vCPU preemptions
+ * in the host. So it is better not to use this CPU.
+ *
+ * Since task is running, call a stopper to push the task out. This is
+ * similar to how task moves during hotplug. In select_fallback_rq a
+ * preferred CPU will be chosen and henceforth task shouldn't come back to
+ * this CPU again.
+ *
+ * Works for FAIR class only
+ *
+ * If task is affined only non-preferred CPUs, it can't be moved out
+ */
+void sched_push_current_non_preferred_cpu(struct rq *rq)
+{
+	struct task_struct *push_task = rq->curr;
+
+	/* Preferred feature works only for FAIR class */
+	if (push_task->sched_class != &fair_sched_class)
+		return;
+
+	if (kthread_is_per_cpu(push_task) ||
+	    is_migration_disabled(push_task))
+		return;
+
+	/* Don't push the task if it is affined only on non preferred CPUs */
+	if (!task_has_preferred_cpus(push_task))
+		return;
+
+	/* There is already a stopper thread for this. Dont race with it. */
+	if (rq->push_task_work_done == 1)
+		return;
+
+	/* sched_tick runs with interrupts disabled. */
+	get_task_struct(push_task);
+
+	scoped_guard (rq_lock, rq)
+		rq->push_task_work_done = 1;
+
+	stop_one_cpu_nowait(rq->cpu, sched_non_preferred_cpu_push_stop,
+			    push_task, this_cpu_ptr(&npc_push_task_work));
+}
+#endif
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 36ae20310891..711fc8bd7ebc 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1277,6 +1277,8 @@ struct rq {
 
 	struct list_head cfs_tasks;
 
+	bool			push_task_work_done;
+
 	struct sched_avg	avg_rt;
 	struct sched_avg	avg_dl;
 #ifdef CONFIG_HAVE_SCHED_AVG_IRQ
@@ -4239,4 +4241,10 @@ static inline bool task_has_preferred_cpus(struct task_struct *p)
 	return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
 }
 
+#ifdef CONFIG_PREFERRED_CPU
+void sched_push_current_non_preferred_cpu(struct rq *rq);
+#else	/* !CONFIG_PREFERRED_CPU */
+static inline void sched_push_current_non_preferred_cpu(struct rq *rq) { }
+#endif
+
 #endif /* _KERNEL_SCHED_SCHED_H */
-- 
2.47.3